1. Field of the Invention
The present invention relates to an antistatic polyester film and a process for producing the same. More particularly, the invention relates to an antistatic polyester film having excellent antistatic property under low humidity, thus having antistatic property substantially having no humidity dependency, which is useful in packaging materials, photographic materials, electronic materials, graphic materials, plate-making film, magnetic cards (for example, telephone cards and prepaid cards), OHP films, magnetic recording materials (for example, magnetic tapes such as audio tapes or video tapes, and magnetic disks such as floppy disks), printing materials, and the like, and a process for producing the same.
2. Descriptions of the Related Art
Polyester film are widely used in base films of, for example, packaging materials, photographic materials, magnetic cards, magnetic recording media and printing materials. However, as the common problems in plastic films, the polyester films have the disadvantages that static electricity tends to occur, and various troubles tend to occur in plate-making step, processing step and use of products.
As one of the methods for preventing the problems due to electrification of those films, various methods for forming an antistatic coating film on the surface of a film have been proposed and have been put in practical use. As antistatic agents to be contained in this antistatic coating film, electron-conductive materials represented by ion-conductive type of low- or high-molecular-weight organic materials, conductive metal oxides and conductive organic polymers have hitherto been known. However, those materials have advantages and disadvantages, respectively. Therefore, an appropriate antistatic agent is used depending on its purpose of use in view of the characteristic thereof.
For example, surface active agent type anionic antistatic agents such as long chain alkyl compounds having a sulfonate group (Japanese Patent Application Laid-open No. Hei 4-28728 (Hereinafter simply referred to as "JP-A")) are known as an antistatic agent of low molecular weight organic material. Further, polymers having Ionized nitrogen atom in the main chain (JP-A-3-255139, 4-288127 and 6-172562) and sulfonate-modited polystyrenes (JP-A-5-820890) are known as an antistatic agent of high molecular weight organic material.
However, antistatic coating layers using an ion-conductive type antistatic agent of low molecular weight organic material have the problems that a part of an antistatic agent moves in the coating layer to concentrate the same on an interface, and transfers to the opposite surface of the film, and the antistatic property deteriorates with the passage of time.
On the other hand, antistatic coating layers using an ion-conductive type antistatic agent of high molecular weight organic material require additions of an antistatic agent in high proportion in order to develop good antistatic property, and also require to form an antistatic coating layer having a large thickness, which are not economical. Further, in the case where the ion-conductive antistatic agent is used, its antistatic property depends on humidity, and the antistatic performance completely deactivates under dry condition.
On the other hand antistatic coating layers using an electron-conductive type antistatic agent represented by conductive metal oxides and conductive organic polymers also require the addition of an antistatic agent in the high Proportion in order to develop good antistatic property, and require to form an antistatic coating layer having a large thickness, which are not economical. Further, in the case where the conductive metal oxide is used alone, if an aromatic polyester film and the antistatic coating layer are subjected to, for example, stretching processing, a distance between mutual conductive oxide particles increases, so that an antistatic property lowers.